CD28 and CTLA-4 coreceptor expression and signal transduction.

Abstract

T-cell activation is mediated by antigen-specific signals from the TCRzeta/CD3 and CD4-CD8-p56lck complexes in combination with additional co-signals provided by coreceptors such as CD28, inducible costimulator (ICOS), cytotoxic T-lymphocyte antigen-4 (CTLA-4), programmed death (PD-1), and others. CD28 and ICOS provide positive signals that promote and sustain T-cell responses, while CTLA-4 and PD-1 limit responses. The balance between stimulatory and inhibitory co-signals determines the ultimate nature of T-cell responses where response to foreign pathogen is achieved without excess inflammation and autoimmunity. In this review, we outline the current knowledge of the CD28 and CTLA-4 signaling mechanisms [involving phosphatidylinositol 3 kinase (PI3K), growth factor receptor-bound protein 2 (Grb2), Filamin A, protein kinase C theta (PKCtheta), and phosphatases] that control T-cell immunity. We also present recent findings on T-cell receptor-interacting molecule (TRIM) regulation of CTLA-4 surface expression, and a signaling pathway involving CTLA-4 activation of PI3K and protein kinase B (PKB)/AKT by which cell survival is ensured under conditions of anergy induction.

CD28 and CTLA-4 associate with various intracellular signaling proteins. CD28 associates with PI3K and Grb2 via SH2 domain binding to the Tyr–Val–Asn–Met (YMNM) motif. PI3K generates PI3,4P2 and PI3,4,5P3 lipids, while Grb2 binds to the exchange factor Sos1, an activator of the GTPase p21ras. CD28–Grb2 binding is also needed for the phosphorylation and activation of Vav1. In turn, Vav1 can activate Rac1 that activates the serine/threonine kinase JNK, may activate Cdc42 in the activation of WASP, and is needed for the membrane association of PKCθ. Protein tyrosine phosphatase 1 (SHP-1) can also associate with Vav1, Grb2, and Sos1, dampening signals from CD28. The CD28–PYAPP motif (located further to the C-terminus) has been reported to bind to FLNA and colocalizes with PKCθ and CD28. Mutations in the PYAPP motif block PKCθ formation in a cSMAC. CD28 endocytosis is regulated by the combination of mediators, PI3K, WASP, and SNX9. Mutations that disrupt PI3K binding impair internalization via a clathrin-dependent mechanism. SNX9 binds WASP via its SH3 domain and uses its PX domain to interact with the p85 subunit of PI3K and its product PIP3. WASP, SNX9, PI3K, and CD28 colocalize within clathrin-containing endocytic vesicles after TCR/CD28 costimulation. On the other hand, CTLA-4 binds to PI3K using its YVKM motif as well as phosphatases SHP-2 and PP2A. Phosphatases have been postulated to generate negative signals. CTLA-4 can also block the expression of lipid rafts (GEMs) and the induction by the TCR of ZAP-70 microcluster formation. CTLA-4, cytotoxic T-lymphocyte antigen-4; PI3K, phosphatidylinositol 3-kinase; SH2, src homology 2; WASP, Wiskott–Aldrich syndrome protein; PKCθ, protein kinase C h; cSMAC, central supramolecular activation cluster; SNX, sorting nexin; TCR, T-cell receptor; YVKM, Val–Tyr–Val–Lys–Met; SHP-1, SH2-domain-containing protein tyrosine phosphatase 1; GEM, glycolipid-enriched microdomain.

Newly synthesized CTLA-4 binds to the transmembrane adapter TRIM in the TGN promoting the formation of CTLA-4-containing vesicles and their transport to the cell surface. On the cell surface, CTLA-4 and TRIM no longer associate allowing TRIM to interact with other receptors, possibly the TCR complex. CTLA-4 externalization is also dependent on general factors such as PLD and GTPase ARF-1. Shuttling to the lysosomal compartment from the TGN occurs due to adapter AP-1 binding to GVY201VKM motif in CTLA-4. On the surface, CTLA-4 becomes phosphorylated on the Y201VKM by kinases Lck, Fyn, and Rlk leading to the association of PI3K and possibly other proteins. Phosphorylation retards internationalization. Dephosphorylation allows binding to the clathrin adapter AP-2 to the GVY201VKM motif and rapid internalization to endosomes and lysosomes. Upon T-cell activation, CTLA-4 enriched lysosomes (and endosomes) are recycled to the cell surface. This process involves Wortmannin-sensitive lipid kinases but not PI3K (). CTLA-4, cytotoxic T-lymphocyte antigen-4; TRIM, T-cell receptor-interacting molecule; TGN, trans-Golgi network; TCR, T-cell receptor; PLD, phospholipase D; ARF, adenosine diphosphate ribosylation factor; AP, adapter protein; PI3K, phosphatidylinositol 3-kinase.